This invention relates to metal catalysts for alkylene oxide polymerization.
Alkylene oxides such as ethylene oxide, propylene oxide and 1,2-butylene oxide are polymerized to form a wide variety of polyether products. For example, polyether polyols are prepared in large quantities for polyurethane applications. Other polyethers are used as lubricants, brake fluids, compressor fluids, and many other applications.
These polyethers are commonly prepared by polymerizing one or more alkylene oxides in the presence of an initiator compound and an alkali metal catalyst. The initiator compound is typically a material having one or more hydroxyl, primary or secondary amine, carboxyl or thiol groups. The function of the initiator is to set the nominal functionality (number of hydroxyl groups/molecule) of the product polyether, and in some instances to incorporate some desired functional groups into the product.
Until recently, the catalyst of choice was an alkali metal hydroxide such as potassium hydroxide. Potassium hydroxide has the advantages of being inexpensive, adaptable to the polymerization of various alkylene oxides, and easily recoverable from the product polyether.
However, to a varying degree, alkali metal hydroxides catalyze an isomerization of propylene oxide to form allyl alcohol. Allyl alcohol acts as a monofunctional initiator during the polymerization of propylene oxide. Thus, when potassium hydroxide is used to catalyze propylene oxide polymerizations, the product contains allyl alcohol-initiated, monofunctional impurities. As the molecular weight of the product polyether increases, the isomerization reaction becomes more prevalent. Consequently, poly(propylene oxide) products having equivalent weights of about 800 or more tend to have very significant quantities of the monofunctional impurities when prepared using KOH as the catalyst. This tends to reduce the average functionality and broaden the molecular weight distribution of the product.
More recently, the so-called double metal cyanide (DMC) catalysts have been used commercially as polymerization catalysts for alkylene oxides. These DMC catalysts are described, for example, in U.S. Pat. Nos. 3,278,457, 3,278,458, 3,278,459, 3,404,109, 3,427,256, 3,427,334, 3,427,335 and 5,470,813, among many others. Those DMC catalysts that are active usually do not significantly promote the isomerization of propylene oxide, polyethers having low unsaturation values and higher molecular weights can be prepared, compared to potassium hydroxide-catalyzed polymerizations. Recently, developmental and commercial efforts have focused almost exclusively on zinc hexacyanocobaltate, together with a specific completing agent, t-butanol.
As described in U.S. Pat. No. 5,470,813, one disadvantage of DMC catalysts is that they tend to require an induction period of close to an hour to many hours in some cases before becoming active. Little polymerization occurs during this induction period, but it is followed by a strongly exothermic reaction. For some operations, it would be desirable to reduce this induction period and to provide a less strongly exothermic reaction.
It would be desirable, therefore, to provide an active catalyst for polymerizing alkylene oxides that exhibits a short induction period before rapidly polymerizing alkylene oxides, and provides for a more controlled exotherm when the rapid polymerization commences.
In one aspect, this invention is a metal cyanide catalyst complexed with an organic sulfone (R5xe2x80x94S(O)2xe2x80x94R5) or sulfoxide (R5xe2x80x94S(O)xe2x80x94R5) compound.
In another aspect, this invention is an improvement in a process for polymerizing an epoxide compound in the presence of a catalyst, the improvement wherein the catalyst is a metal cyanide catalyst complexed with an organic sulfone or sulfoxide compound.
It has been found that the metal cyanide catalyst complex of the invention has excellent activity as an epoxide polymerization catalyst. In particular, the catalyst often exhibits sharply reduced induction periods when used in such polymerizations, compared, for example, to the zinc hexacyanocobaltate/t-butanol/poly(propylene oxide) complex that is most commonly used. In addition, smaller, more easily controlled exotherms are usually seen when rapid alkylene oxide polymerization begins.
By xe2x80x9cmetal cyanide catalystxe2x80x9d, it is meant a catalyst represented by the formula
Mb[M1(CN)r(X)t]c[M2(X)6]dxe2x80xa2zLxe2x80xa2aH2Oxe2x80xa2nM3xAy 
wherein
M is a metal ion that forms an insoluble precipitate with the M1(CN)r(X)t group and which has at least one water soluble salt;
M1 and M2 are transition metal ions that may be the same or different;
each X independently represents a group other than cyanide that coordinates with an M1 or M2 ion;
M3xAy represents a water-soluble salt of metal ion M3 and anion A, wherein M3 is the same as or different than M;
L represents the organic sulfone or sulfoxide compound;
b and c are positive numbers that, together with d, reflect an electrostatically neutral complex;
d is zero or a positive number;
x and y are numbers that reflect an electrostatically neutral salt;
r is from 4 to 6; t is from 0 to 2; and
a and n are positive numbers (which may be fractions) indicating the relative quantities of water sulfone or sulfoxide compound, and M3xAy, respectively.
The X groups in any M2(X)6 do not have to be all the same. The molar ratio of c:d is advantageously from about 100:0 to about 20:80, more preferably from about 100:0 to about 50:50, and even more preferably from about 100:0 to about 80:20.
Similarly, mixtures of two or more different M1(CN)r(X)t groups can be used.
M and M3 are preferably metal ions selected from the group consisting of Zn+2, Fe+2, Co+2, Ni+2, Mo+4, Mo+6, Al+3, V+4, V+5, Sr+2, W+4, W+6, Mn+2, Sn+2, Sn+4, Pb+2, Cu+2, La+3 and Cr+3. M and M3 are more preferably Zn+2, Fe+2, Co+2, Ni+2, La+3 and Cr+3. M is most preferably Zn+2.
M1 and M2 are preferably Fe+3, Fe+2, Co+3, Co+2, Cr+2, Cr+3, Mn+2, Mn+3, Ir+3, Ni+2, Rh+3, Ru+2, V+4 and V+5. Among the foregoing, those in the plus-three oxidation state are more preferred. Co+3 and Fe+3 are even more preferred and Co+3 is most preferred. M1 and M2 may be the same or different.
Preferred groups X include anions such as halide (especially chloride), hydroxide, sulfate, carbonate, late, thiocyanate, isocyanate, isothiocyanate, C1-4 carboxylate and nitrite (NO2xe2x88x92), and uncharged species such as CO, H2O and NO. Particularly preferred groups X are NO, NO2xe2x88x92 and CO.
r is preferably 5 or 6, most preferably 6 and t is preferably 0 or 1, most preferably 0. In most instances, r+t will equal 6.
Suitable anions A include halides such as chloride and bromide, nitrate, sulfate, carbonate, cyanide, oxalate, thiocyanate, isocyanate, isothiocyanate, perchlorate and C1-4 carboxylate. Chloride ion is especially preferred.
L represents an organic sulfone or sulfoxide compound. Suitable sulfone compounds are represented by the general formula R5xe2x80x94S(O)2xe2x80x94R5, where each R5 is unsubstituted or inertly substituted alkyl, cycloalkyl, aryl, or, together with the other R5, forms part of a ring structure that includes the sulfur atom of the sulfone (xe2x80x94S(O)2xe2x80x94) group. Suitable sulfoxide compounds are represented by the general formula R5xe2x80x94S(O)xe2x80x94R5, where each R5 is as just described. In this context, xe2x80x9cinertly substitutedxe2x80x9d means that the group contains no substituent which undesirably reacts with the metal cyanide compound, its precursor compounds (as described below) or an alkylene oxide, or which otherwise undesirably interferes with the polymerization of an alkylene oxide. Examples of such inert substituents include ether, alkoxyl, hydroxyl, nitrite, aldehyde, ketone, amide, sulfide, additional sulfone or sulfoxide groups, and the like. Each R5 is preferably unsubstituted and is also preferably either an alkyl group or, together with the other R5, forms part of a ring structure that includes the sulfone or sulfoxide group. Especially preferred R5 groups are 1-4 carbon atom alkyl groups or those that together form a 5-8 member ring with the sulfur atom of the sulfone or sulfoxide groups. More preferred compounds are water-soluble, including for example, dimethyl sulfoxide (DMSO), tetramethylene sulfoxide, 2,2-sulfonyl diethanol, dimethyl sulfone and sulfolane (tetramethylene sulfone). DMSO is the most preferred compound, because it exhibits particularly short induction periods in initiated propylene oxide polymerization.
The sulfone or sulfoxide compound is generally and preferably the sole completing agent.
The catalyst complex is conveniently made by first dissolving or dispersing a water-soluble metal cyanide compound in an inert solvent such as water or methanol. Mixtures of two or more metal cyanide compounds can be used. The water-soluble metal cyanide compound is represented by the general formula Bu[M1(CN)r(X)t]v, in which B is hydrogen or a metal that forms a water-soluble salt with the [M1(CN)r(X)t] ion, u and v are integers that result in an electrostatically neutral compound and M1, X, r and t are as described before. B is preferably hydrogen, sodium or potassium. Compounds in which B is hydrogen are conveniently formed by passing an aqueous solution of the corresponding alkali metal salt through a cation-exchange resin that is in the hydrogen form.
In addition, the solution or dispersion of the metal cyanide compound may also contain compounds that have the structure Bu[M2(X)6]v, wherein M2 is a transition metal and X, B, u and v are as before. M2 may be the same as or different from M1.
The solution or dispersion is then combined the resulting solution(s) with an aqueous solution of a water soluble metal salt, in the presence of the sulfone or sulfoxide compound. The metal salt is represented by the general formula MxAy, where M, A, x and y are as defined before. Especially suitable metal salts include zinc halides, zinc hydroxide, zinc sulfate, zinc carbonate, zinc cyanide, zinc oxalate, zinc thiocyanate, zinc isocyanate, zinc C1-4 carboxylates, and zinc nitrate. Zinc chloride is most preferred.
The temperature of mixing is not critical, provided that the starting materials remain in solution or well dispersed until the mixing is performed. Temperatures of about 10 to about the boiling point of the inert solvent, particularly 15-40xc2x0 C., are most suitable. The mixing can be done with rapid agitation. Intimate mixing techniques as are described in U.S. Pat. No. 5,470,813 can be used, but are not necessary.
In precipitating the catalyst, at least enough metal salt is used to provide one equivalent of metal ion (M) for each equivalent of metal cyanide ion (M1(CN)r(X)t), plus each equivalent of M2(X)6 ion, if used. It has been found that in general, more active catalysts are those prepared using an excess of the metal salt. This excess metal is believed to exist in the catalyst complex as a salt in the form MxAy or M3xAy. This excess metal salt can be added in the precipitation step, such as by adding up to about three equivalents of metal salt, preferably from about 1.1 to about 3, more preferably about 1.5 to about 2.5 equivalents of metal salt, per combined equivalents of metal cyanide ion plus any M2(X)6 ions.
An alternate way to add the excess metal salt is to do so in a separate step following the precipitation step, as described more fully below. The metal ion in excess salt may be different than that in the metal salt used to precipitate the catalyst.
It is preferred to add the solution of the metal cyanide compound to that of the metal salt, and it is also preferred that the mixing be done with agitation. Agitation is preferably continued for a period after the mixing is completed. The metal cyanide catalyst precipitates and forms a dispersion in the supernatant fluid.
The catalyst complex may be precipitated by mixing the solution or dispersion of the metal salt with the solution or dispersion of the metal cyanide compound in the presence of the sulfone or sulfoxide compound. One way of doing this is to add the sulfone or sulfoxide compound to the solution or dispersion of the metal cyanide compound before the solutions are mixed. Alternately, both starting solutions or dispersions may be added simultaneously with the sulfone or sulfoxide compound. A third way is to mix the starting solutions or dispersions, followed immediately by adding the sulfone or sulfoxide compound. After adding this initial amount of sulfone or sulfoxide compound, the mixture is generally stirred for several minutes to allow the desired catalyst complex to form and precipitate.
The resulting precipitated catalyst complex is then recovered by a suitable technique such as filtration or centrifugation. Preferably, the catalyst complex is subjected to one or more subsequent washings with water, sulfone or sulfoxide compound, polyether polyol (when used) or some combination thereof. This is conveniently done by re-slurrying the catalyst in the liquid with agitation for several minutes and filtering. Washing is preferably continued at least until essentially all unwanted ions, particularly alkali metal and halide ions, are removed from the complex.
It has been found that catalyst preparation is sometimes easier if the catalyst is treated with a polyether polyol of a molecular weight of about 300-4000. When a polyether polyol is used in the catalyst complex, it can be added with the initial amount of sulfone or sulfoxide compound, or in one or more subsequent washings of the complex.
The final catalyst complex is conveniently dried, preferably under vacuum and moderately elevated temperatures (such as from about 50-60xc2x0 C.) to remove excess water and volatile organics. Drying is preferably done until the catalyst complex reaches a constant weight.
In an alternative technique for forming the catalyst complex, an aqueous solution containing only a stoichiometric amount of metal salt in relation to the combined amount of metal cyanide compound (and any M2(X)6 compound that is used) is used in the initial mixing and precipitation step. After this initial precipitation is complete, the precipitate is washed with water to remove unwanted ions. The precipitate is then combined with a small amount of a solution containing water, additional metal salt, and the sulfone or sulfoxide compound. The metal salt used may the same as that used in forming the precipitate, or may be a salt of a different metal. The amount of this added solution is preferably that amount which is absorbed by the precipitate. A typical amount of solution to be used is from about 0.5 to about 2, preferably about 0.8 to about 1.5, more preferably about 1 to about 1.5 milliliters of solution per gram of isolated precipitate. The amount of metal salt added with this solution is advantageously about 9 to about 30, preferably about 11 to about 25, parts by weight per 100 parts by weight of the isolated precipitate. The sulfone or sulfoxide compound is advantageously present in a weight ratio of about 90:10 to about 10:90, preferably about 70:30 to about 30:70, with the water. If desired, a polyether polyol can be included in the solution. The resulting catalyst complex can be dried and used without further treatment, or may be subjected to additional washings with water as before, although it is preferred not to perform additional washings with sulfone compound, sulfoxide compound or polyether polyol.
The catalyst complex of the invention is used to polymerize alkylene oxides to make polyethers. In general, the process includes mixing a catalytically effective amount of the catalyst with an alkylene oxide under polymerization conditions, and allowing the polymerization to proceed until the supply of alkylene oxide is essentially exhausted. The concentration of the catalyst is selected to polymerize the alkylene oxide at a desired rate or within a desired period of time. Generally, a suitable amount of catalyst is from about 5 to about 10,000 parts by weight metal cyanide catalyst per million parts combined weight of alkylene oxide, and initiator and comonomers, if present. More preferred catalyst levels are from about 10, especially from about 25, to about 1000, more preferably about 250 ppm, on the same basis.
To control molecular weight, impart a desired functionality (number of hydroxyl groups/molecule) or a desired terminal functional group, an initiator compound as described before is preferably mixed with the catalyst complex at the beginning of the reaction. Suitable initiator compounds include monoalcohols such methanol, n-propanol, isopropanol, n-butanol, isobutanol, t-butanol, octanol, octadecanol, 3-butyn-1-ol, 3-butene-1-ol, propargyl alcohol, 2-methyl-2-propanol, 2-methyl-3-butyn-2-ol, 2-methyl-3-butene-2-ol, 3-butyn-1-ol, 3-butene-1-ol and the like. The suitable monoalcohol initiator compounds include halogenated alcohols such as 2-chloroethanol, 2-bromoethanol, 2-chloro-1-propanol, 3-chloro-1-propanol, 3-bromo-1-propanol, 1,3-dichloro-2-propanol, 1-chloro-2-methyl-2-propanol as well as nitroalcohols, keto-alcohols, ester-alcohols, cyanoalcohols, and other inertly substituted alcohols. Suitable polyalcohol initiators include ethylene glycol, propylene glycol, glycerine, 1,1,1-trimethylol propane, 1,1,1-trimethylol ethane, 1,2,3-trihydroxybutane, pentaerythritol, xylitol, arabitol, mannitol, 2,5-dimethyl-3-hexyn-2,5-diol, 2,4,7,9-tetramethyl-5-decyne-4,7-diol, sucrose, sorbitol, alkyl glucosides such a methyl glucoside and ethyl glucoside and the like. Low molecular weight polyether polyols, particularly those having an equivalent weight of about 350 or less, more preferably about 125-250, are also useful initiator compounds.
Among the alkylene oxides that can be polymerized with the catalyst complex of the invention are ethylene oxide, propylene oxide, 1,2-butylene oxide, styrene oxide, and mixtures thereof Various alkylene oxides can be polymerized sequentially to make block copolymers. More preferably, the alkylene oxide is propylene oxide or a mixture of propylene oxide and ethylene oxide and/or butylene oxide. Especially preferred are propylene oxide alone or a mixture of at least 50 weight % propylene oxide and up to about 50 weight % ethylene oxide.
In addition, monomers that will copolymerize with the alkylene oxide in the presence of the catalyst complex can be used to prepare modified polyether polyols. Such comonomers include oxetanes as described in U.S. Pat. Nos. 3,278,457 and 3,404,109, and anhydrides as described in U.S. Pat. Nos. 5,145,883 and 3,538,043, which yield polyethers and polyester or polyetherester polyols, respectively. Hydroxyalkanoates such as lactic acid, 3-hydroxybutyrate, 3-hydroxyvalerate (and their dimers), lactones and carbon dioxide are examples of other suitable monomers that can be polymerized with the catalyst of the invention.
The polymerization reaction typically proceeds well at temperatures from about 25 to about 150xc2x0 C., preferably from about 80-130xc2x0 C. A convenient polymerization technique involves mixing the catalyst complex and initiator, and pressuring the reactor with the alkylene oxide. After a short induction period, polymerization proceeds, as indicated by a loss of pressure in the reactor. Once the polymerization has begun, additional alkylene oxide is conveniently fed to the reactor on demand, until enough alkylene oxide has been added to produce a polymer of the desired equivalent weight.
Another convenient polymerization technique is a continuous method. In such continuous processes, an activated initiator/catalyst mixture is continuously fed into a continuous reactor such as a continuously stirred tank reactor (CSTR) or a tubular reactor. A feed of alkylene oxide is introduced into the reactor and the product continuously removed.
The catalyst of this invention is especially useful in making propylene oxide homopolymers and random copolymers of propylene oxide and up to about 15 weight percent ethylene oxide (based on all monomers). The polymers of particular interest have a hydroxyl equivalent weight of from about 800, preferably from about 1000, to about 5000, preferably about 4000, more preferably to about 2500, and unsaturation of no more than 0.02 meq/g, preferably no more than about 0.01 meq/g.
The product polymer may have various uses, depending on its molecular weight, equivalent weight, functionality and the presence of any functional groups. Polyether polyols so made are useful as raw materials for making polyurethanes. Polyethers can also be used as surfactants, hydraulic fluids, as raw materials for making surfactants and as starting materials for making aminated polyethers, among other uses.
The following examples are provided to illustrate the invention, but are not intended to limit its scope. All parts and percentages are by weight unless otherwise indicated.